Sep 16, 2011

Stone heat storage system cuts greenhouse costs

Technology developed in Thailand could have an energy-saving application for North American greenhouses.

A University of Guelph researcher said the system employs the use of an underground stone heat sink in combination with solar thermal collectors and a biomass gasification furnace. Mathias Leon spoke at the Growing the Margins Conference in London, Canada in March.

“It was a development project … funded by the British government,” Leon said. “The purpose for that system was for drying fruit and vegetables, but I could see the idea working very well for Ontario’s greenhouse industry.”

The idea is to collect the heat energy from both the panels and furnace over the day and release it overnight. The experimental stone storage in Thailand supplied up to 16 hours of continuous heat.

The solar panels should be able to raise the temperature of the stone storage to 40-50˚ C. Combined with a furnace, temperatures as high as 600-700˚ C can be achieved.

The higher temperatures involved are an advantage over water-heat storage systems currently in use by Ontario greenhouse growers. Less space is needed and there are no concerns with leaks, freezing or contamination.

“All the grower would have to do is start and stop the gasifier and feed it with fuel,” Leon said.

The solar thermal system component reduces the amount of fuel required for the biomass furnace. In instances when outdoor temperatures are high enough, solar alone would provide enough heat to charge the storage.

Leon envisions self-sufficient systems requiring no back-up heat source. Solar thermal collection systems are virtually trouble-free, and with large greenhouse operations more than a single biomass gasifier could be installed.

Systems would have a minimal carbon footprint and should reduce energy costs for growers, Leon said. Electronic controls would adjust temperature and airflow rates.
While the Thai project utilized an aboveground, insulated stone storage, Leon envisions underground storage in Ontario for greenhouse applications. This would protect the storage from the weather.

During the eight years spent on the project in Thailand, many technological questions – such as those related to airflow and stone size – were answered.

Leon said that while there are thermal rock storage systems in Europe, his design is unique. He’s currently looking for cooperators interested in the approach.

By Jeffrey Carter, Ontario correspondent